Recently, a magnetic recording medium such as one for data storage or digital video tapes becomes a high-density and high-capacity magnetic recording medium. Generally, as recording systems used for such a magnetic recording medium, there are a helical scan type recording system as shown in FIG. 1 and a linear recording system as shown in FIG. 2. In the helical scan type recording system, a tape is pulled out from a cartridge having two reels, a cylindrical head rotating at a high speed is scanned, and the reading/writing is carried out obliquely relative to the tape. In the linear recording system, a tape is guided to a head from a cartridge having one reel, and the reading/writing is carried out at straight lines relatively to the tape in the longitudinal direction. Although the linear recording system is low in recording density as compared with the helical scan type recording system, because the winding of the tape relative to the head is loose, damage to the tape is little, and it has a high reliability for data storage.
Recently, in LTO (Linear Tape Open) or SDLT (Super Digital Linear Tape) employing such a linear recording system, one having a high capacity of 100 GB or more per one reel has been developed.
For making the capacity high, generally there are three methods of a method for increasing the number of tracks, a method for setting a recording wavelength at a shorter wavelength, and a method for increasing a tape length.
If the number of tracks is increased, because the width of one track becomes small, it becomes important to control the dimensional stability in the transverse direction of the tape. Further, in order to realize a sufficient electromagnetic conversion property at a condition where the recording wavelength is set at a shorter wavelength, a surface flatness is required. Furthermore, because the size of a cartridge for a magnetic recording medium is basically unchangeable, in order to increase the tape length per one reel, it is necessary to make the tape thinner and accompanying with it to increase the tape strength. From these three points of view, various investigations have been conducted. JP-A-2002-329312, WO/00-076749 and JP-A-2003-67913 describe as to optimization of thermal expansion coefficient and humidity expansion coefficient in the transverse direction of a tape, making the diameter of added particles small, and making a base film strong by increase of a draw ration of stretching or by reinforcement with a metal. However, even if these technologies are employed, a sufficient electromagnetic conversion property has not been able to be obtained for a magnetic recording medium of linear recording system which has a high capacity of 100 GB or more per one reel. As a result so far, it has been clarified that deterioration of microplanarity of the film surface ascribed to high-temperature storage or heat loading during film processing affects the properties of the magnetic recording medium.
Generally in the linear recording system, because the winding of the tape is loose and a force of the head for pushing the tape in its perpendicular direction is weak, a spacing loss is liable to occur as compared with a case of helical recording system. The spacing loss is a nanometer-order gap generated between the head and the tape, and if the spacing loss is great, the electromagnetic conversion property deteriorates. For achieving a high capacity, it is required to make the recording wavelength of a magnetic recording medium small to be lower than 0.6 μm and control the surface more finely. However, in the known art, the investigation is still carried out with properties representing average values of the whole of a film such as a thermal expansion coefficient, a humidity expansion coefficient, a strength and a degree of crystallization. The control of the planarity at a micro level, which is a subject at the present time, has not been able to be carried out.